The present invention relates generally to the generation of monocrystalline materials and, more particularly, to a method and apparatus for growing monocrystalline silicon carbide.
Silicon carbide (SiC) is wide-band-gap semiconductor material that has a number of characteristics that make it an ideal candidate for a variety of semiconductor applications including, but not limited to, light sources, power diodes, field-effect transistors, and photodiodes. The ability to realize the benefits offered by SiC is largely controlled by the purity of the material as well as its structural characteristics.
The methods most commonly used in producing SiC single crystals are sublimation techniques based on the Lely method, this method utilizing vapor-phase crystallization of evaporated solid silicon carbide. (See, for example, U.S. Pat. Ser. Nos. 2,854,364 and 4,866,005). As shown by Karpov et al. in an article entitled Excess Phase Formation During Sublimation Growth of Silicon Carbide, 6th Int. Conf. on Silicon Carbide, Kyoto, Japan, 74-75 (September 1995), in order to achieve SiC monocrystalline growth from vapor without forming secondary-phase inclusions, the external silicon (Si) flux on the growing surface must exceed the carbon (C) flux. The ability to achieve the desired excess silicon flux depends on the temperature of the growing surface and, in the case of sublimation techniques, the composition of the vapor adjacent to the growth surface.
As silicon molecules have the maximum concentration in the gaseous phase, any drift of the substance from the growth zone will result in the vapor phase within the growth zone being depleted of silicon and enriched with carbon. Excessive carbon in the growth zone leads to source graphitization, crystal quality degradation, and eventually the discontinuation of the growth process. On the other hand, excessive silicon in the growth zone may result both in the formation of defects on the growing surface of the SiC crystal, primarily due to the precipitation of excess silicon drops, and in the generation of polytypes differing from the seed polytype. Accordingly, it has been established that the best characteristics for the as-grown SiC single crystal are achieved when the vapor composition in the growth zone is shifted the towards the vapor phase corresponding to the SiCxe2x80x94Si system.
U.S. Pat. Ser. No. 2,854,364 discloses locating SiC powder with a mass of more than three times the mass of the single crystal to be grown in the growth zone in order to maintain a relatively constant vapor phase composition, the powder serving as a source of silicon carbide vapors. As disclosed, the drift of SiC vapors into the space outside the growth zone is balanced by the generation of SiC vapors from the SiC powder. The duration of the stable growth process is limited by the quantity of SiC powder located in the growth zone. Once the source of SiC vapors becomes depleted, the vapor composition shifts to the non-advantageous SiCxe2x80x94C system.
In U.S. Pat. Ser. No. 4,866,005 a technique is disclosed that continuously feeds small portions of SiC powder into a temperature zone of the growth chamber. Although this technique does allow a SiCxe2x80x94Si system to be maintained indefinitely, it is an inefficient process due to the SiC material consumed in addition to the SiC source as well as the growth zone geometry. As disclosed, the evaporating surface of the SiC vapor source is approximately 10 centimeters from the growing surface of the seed crystal, a distance that far exceeds the Si, Si2C, SiC2 molecular track length at the working pressure in the growth zone.
U.S. Pat. Ser. No. 4,147,572 discloses a growth technique in which the evaporating surface of the SiC source and the growth surface of the SiC seed crystal are arranged in parallel at a distance that is less than 20 percent of the maximum linear dimension of the source. The single crystals are grown in a graphite crucible in an inert gas atmosphere at temperatures of 1600 to 2000xc2x0 C. with an axial thermal gradient of 5 to 200xc2x0 C. per centimeter. This technique is limited to relatively small crystals, typically less than 1 millimeter thick, due to a sharp drop in the growth rate as the crystallization time increases. The change in growth rate is due to the silicon at the edge of the growth zone being volatilized, thereby causing excessive carbon to be released from the evaporating surface of the SiC source and the growing surface of the grown crystal. Single crystals obtained by this technique show defects such as secondary-phase inclusions (predominantly, graphite), micropipes with a density of more than 100 per square centimeter, and dislocations of at least 104 per square centimeter. These crystals also have relatively high concentrations of residual impurities such as boron, oxygen, etc.
In an article by D. Hofmann et al. entitled The Use of Tantalum Container Material for Quality Improvement of SiC Crystals Grown by the Sublimation Technique, 6th Int. Conf. on Silicon Carbide, Kyoto, Japan, 15 (September 1995), it was shown that the inclusion of tantalum (Ta) during the sublimation growth of monocrystalline SiC resulted in the vapor medium produced in the growth zone being close to the SiCxe2x80x94Si system. The favorable aspects were found to occur both in an inert gas atmosphere and in vacuum. Unfortunately it was also found that during the early stages of growth, secondary-phase inclusions of tantalum or its compounds were formed. An increased concentration of dissolved tantalum in the monocrystalline SiC was also noted. Lastly, due to the carbon enrichment of the vapor phase that results from silicon drifting outside of the growth zone, carbon dust was embedded into the growing crystal, further reducing the quality of the growing crystal while simultaneously decreasing the transferal efficiency of source material to the growing crystal.
Another problem associated with the use of a Ta container as disclosed in the previously cited article arises during the initial stage of the growth process when the silicon vapors formed by the evaporating SiC source interact with the material of the tantalum container. As a result of this interaction, a low-melting-point tantalum silicon alloy is formed which can lead to the destruction of the container at the normal growth temperature.
In known sublimation techniques for growing SiC single crystals, the vapor source may be either a pre-synthesized SiC powder of the specified dispersity or a polycrystalline or monocrystalline SiC wafer produced, for example, by the Lely method. Although the use of SiC powder is more economical than the use of wafers, providing a continuous supply of powder into the growth zone, as required to grow large single crystals, is quite complicated. Additionally, SiC powder often includes impurities such as graphite or other dust that are transported to the growth surface along with the SiC molecules. These impurities lead to a high density of micropipes and dislocations in the growing crystal, thus substantially impacting the crystal quality.
Accordingly, what is needed in the art is a method and system that allows high quality SiC single crystals to be grown. The present invention provides such a method and system.
The present invention provides a method and apparatus for growing low dislocation density single crystal silicon carbide. Utilizing the system of the invention, silicon carbide can be grown with a dislocation density of less than 104 per square centimeter, preferably less than 103 per square centimeter, and more preferably less than 102 per square centimeter. The density of micropipes in the as-grown material is less than 10 per square centimeter. The density of secondary phase inclusions is less than 10 per cubic centimeter and preferably less than 1 per cubic centimeter. Depending upon the construction of the crucible, the concentration of tantalum or niobium impurities is less than 1017 per cubic centimeter.
In accordance with the invention, a SiC source and a SiC seed crystal of the desired polytype are co-located within a crucible with the distance separating the source evaporating surface from the growing surface being comparable to the track length of a SiC molecule. The growth zone is defined by the substantially parallel surfaces of the source and the seed in combination with the sidewalls of the crucible. Prior to reaching the growth temperature, the crucible is evacuated and sealed, either directly or through the use of a secondary container housing the crucible.
In further accordance with the invention, the crucible is comprised of tantalum or niobium that has been specially treated. As a result of the treatment, the inner surfaces of the crucible exhibit a depth variable composition of Taxe2x80x94Sixe2x80x94C or Nbxe2x80x94Sixe2x80x94C that is no longer capable of absorbing SiC vapors as the monocrystalline silicon carbide is grown. Accordingly, during crystal growth the vapor-phase composition within the crucible is close to the SiCxe2x80x94Si system with the partial pressure of Si-vapor slightly higher than that in the SiCxe2x80x94Si system. Additionally, the resultant Taxe2x80x94Sixe2x80x94C or Nbxe2x80x94Sixe2x80x94C material is refractory, thus allowing it to withstand the operating temperatures required to grow the silicon carbide.
The crucible is initially fabricated from tantalum or niobium that is preferably at least 99.9 percent pure. Once the crucible is shaped, it undergoes a series of processing steps to clean the surfaces and remove surface contaminants. A thin, near-surface layer of Taxe2x80x94C or Nbxe2x80x94C is then formed and annealed, resulting in a surface that will not interact with carbon particles. Lastly the crucible is annealed in silicon containing vapor that is diluted by an inert gas, preferably argon, resulting in the formation of a depth variable composition of Taxe2x80x94Sixe2x80x94C or Nbxe2x80x94Sixe2x80x94C on the crucible surfaces.
A further understanding of the nature and advantages of the present invention may be realized by reference to the remaining portions of the specification and the drawings.